1. Technical Field
The present invention relates to an ultrasound sensor and a driving method therefor.
2. Related Art
In the related art, there are ultrasound sensors provided with a substrate in which a space is formed, a diaphragm provided on the substrate so as to block the space, and a piezoelectric element provided on the diaphragm. It is proposed to configure an ultrasound sensor unit having a base in which an opening is formed, a diaphragm that closes the opening provided on the base, and a piezoelectric element provided on the diaphragm corresponding to the opening (refer to JP-A-2010-164331).
In this type of ultrasound sensor, the diaphragm is displaced and pressure fluctuations accompanying the displacement occur in the medium (acoustic matching layer or air layer) in the space by the piezoelectric element being displaced with the application of a voltage. In so doing, ultrasound (transmitted ultrasound) is transmitted. When ultrasound (reflected ultrasound) reflected by the measurement target is received and pressure fluctuations occur in the medium in the space, the pressure fluctuations are transmitted to displace the diaphragm, and the piezoelectric element is displaced accompanying the displacement of the diaphragm. In so doing, a voltage is obtained from the piezoelectric element. Information pertaining (position, shape or the like) to the measurement target is detected based on the voltage, that is, the waveform of the transmitted ultrasound or the reflected ultrasound.
Although the ultrasound sensor unit in JPA-2010-164331 is a type (a so-called CAV plane-type) in which the opposite side of the diaphragm to the piezoelectric element becomes a pass-through region of the ultrasound, there are also types (a so-called ACT plane-type) of ultrasound sensor in which the piezoelectric element side of the diaphragm becomes the pass-through region of the ultrasound. Ultrasound sensors are classified into a dedicated transmission-type optimized for transmission of ultrasound, a dedicated reception-type optimized for reception of ultrasound, and a transceiving integrated-type optimized for both transmission and reception of ultrasound, and the like.
In any of the types of ultrasound sensor, polarization occurs in the piezoelectric layer by a voltage being applied to the piezoelectric element during transmission of the transmitted ultrasound, and an electromechanical conversion capacity is exhibited according to the state of the polarization. At this time, the higher the state of the compliance (ease of displacement with respect to the stress in the film thickness direction) of the diaphragm, the more advantageous in achieving improvement in the displacement efficiency of the diaphragm.
However, in JP-A-2010-164331, when polarization is caused in the piezoelectric layer, the stress state that acts on the piezoelectric element according to the polarization changes, and, thereafter, there is a possibility for the piezoelectric element and the diaphragm to be maintained in a state of being convexly bent (that is, downwardly convex) to the space side. When the diaphragm is bent downwardly convex, the compliance when the diaphragm is displaced is lower with respect to a case of being bent upwardly convex. In a case where the diaphragm is displaced only in the region in which the compliance of the diaphragm is low, it becomes difficult to achieve improvements in the transmission efficiency and the reception efficiency of ultrasound.
Such a problem is not limited to the ultrasound sensor in JP-A-2010-164331, and is similarly present in ultrasound sensors that use the electromechanical conversion characteristics of the piezoelectric element. That is, this problem is also present in either of the ACT plane-type and the CAV plane-type of ultrasound sensor, and is present in any of the dedicated transmission-type, the dedicated reception-type, and the transceiving integrated-type of ultrasound sensor.